High-voltage energy storage systems may be used to power electric machines in a variety of systems. For instance, output torque from the electric machines may be used to power an input member of a transmission in certain power plants, as well as in hybrid electric, battery electric, and extended-range electric vehicles. However, as the individual battery cells of the energy storage system age and degrade, battery performance parameters such as cell capacity, open circuit voltage, resistance, and state of charge may change relative to calibrated/new values. As a result, battery degradation may be monitored by a designated controller to determine the amount of electrical energy remaining in the various battery cells. From the remaining electrical energy, estimates can be generated of other values, such as a remaining electric operating range for the various vehicles noted above. The estimated electric operating range can be used by an operator for route planning, and/or by an onboard controller to execute powertrain control actions.